Synthesis and evaluation of N-[11C]methylated analogues of epibatidine as tracers for positron emission tomographic studies of nicotinic acetylcholine receptors.

Four halogen-substituted analogues of N-methylepibatidine, a nicotinic acetylcholine receptor (nAChR) ligand, were synthesized. They were (+/-)-exo-N-methyl-2-(2-halogeno-5-pyridyl)-7-azabicyclo[2. 2.1]heptanes, where halogeno = F (1a), Cl (2a), Br (3a), I (4a). (+/-)-N-Ethylepibatidine (2b) also was synthesized. The compounds 1a, 2a, 3a, and 4a and their corresponding normethyl analogues 1, 2, 3, and 4 inhibited the in vitro binding of [3H]epibatidine to nAChRs to a similar degree, with affinities in the 27-50 pM range. The binding affinity of N-ethylepibatidine (2b), however, was substantially lower. The N-[11C]methyl derivatives of 1, 2, and 3 were synthesized from high-specific radioactivity [11C]methyl iodide using a high-temperature/high-pressure technique. The corresponding radiolabeled compounds [11C]1a, [11C]2a, and [11C]3a were administrated to mice intravenously. The pattern of regional distribution of the three tracers in the mouse brain following intravenous administration matched those of [3H]epibatidine, [3H]norchloroepibatidine, and (+/-)-exo-2-(2-[18F]fluoro-5-pyridyl)-7-azabicyclo[2.2.1]heptane ([18F]FPH), which are highly specific nAChR probes. The initial brain uptake of the 11C analogues and the acute toxicity of the corresponding authentic nonlabeled compounds appeared to be related to their lipophilicity.

Radiolabeled neuronal nitric oxide synthase inhibitors: synthesis, in vivo evaluation, and primate PET studies.

UNLABELLED: The objectives of this study were to synthesize neuronal nitric oxide synthase (NOS-I)-selective imaging agents based on the 2 potent, selective inhibitors AR-R 17443 [N-(4-((2-((phenylmethyl) (methyl)-amino)ethyl)phenyl)-2-thiophenecarboximidamide)] and AR-R 18512 [(N(2-methyl-1,2,3,4-tetrahydroisoquinoline-7-yl)-2-thiophenecarboxim idamide)] in positron-emitting form and to evaluate regional brain uptake in rodents and primates. METHODS: [11C]AR-R 17443 and [11C]AR-R 18512 were produced by N-alkylation of the corresponding desmethyl precursors using [11C]iodomethane. Regional brain uptake of [11C]AR-R 17443 and [11C]AR-R 18512 was assayed in rats and NOS-I knockout mice, and PET was performed in baboons. Tracer kinetic modeling used a 2-compartment plasma and brain tissue model. RESULTS: Yields of [11C]AR-R 17443 and [11C]AR-R 18512 ranged from 8% to 16% at the end of synthesis, with specific activities of 50-178 GBq/micromol (1,350-4,800 Ci/mmol) at the end of synthesis. In rat cerebellum and cortex at 30 min after injection, [11C]AR-R 17443 showed 1.01 +/- 0.01 and 1.63 +/- 0.12 percentage injected dose per gram (%ID/g) uptake, respectively, whereas [11C]AR-R 18512 showed 0.88 +/- 0.01 and 1.30 +/- 0.07 %ID/g uptake, respectively. Attempts to block tracer uptake by pretreatment with the NOS-I-selective inhibitor 7-nitroindazole or the corresponding unlabeled inhibitor (or desmethyl precursor to AR-R 17443 of similar potency) were unsuccessful. A small but significant (20%) decrease in cerebellar uptake of [11C]AR-R 18512 was present in NOS-I knockout mice compared with control mice. PET of [11C]AR-R 18512 in baboons with concurrent regional cerebral blood flow (rCBF) determination before and after administration of blocker showed dose-related decreases in cerebellar uptake that were greater than or equal to decreases in rCBF. Plasma metabolites accounted for 27% of total activity at 30 min after injection. Kinetic modeling of binding potentials revealed a distribution volume of 334 in cerebral blood that dropped 51% after blocker administration. CONCLUSION: Rodent studies for [11C]AR-R 17443 and [11C]AR-R 18512 showed little evidence of specific NOS-I binding. In baboons, we detected a higher uptake of [11C]AR-R 18512 in the cerebellum than in the cortex (approximately 5%, accounting for decreased rCBF because of blockade), indicating minimal specific binding. Analogs of higher affinity are likely required if this class of agents is to prove viable for PET.

Investigation of angiotensin II/AT1 receptors with carbon-11-L-159,884: a selective AT1 antagonist.

UNLABELLED: Antagonists of the angiotensin II AT1 receptor subtype have been recently introduced for treatment of arterial hypertension and for pharmacological studies of these receptors. The purpose of this work was to label such an antagonist with 11C and test the applicability of the radioligand for PET studies. METHODS: The potent and selective nonpeptide AT1 antagonist L-159,884 was labeled with 11C and injected intravenously into six dogs. Renal accumulation and kinetics of the radioligand were imaged with PET at baseline and after receptor blockade with 1 mg/kg MK-996. Time-activity curves were derived from the renal cortex and were analyzed by the Gjedde-Patlak plot to obtain the influx rate constant of the radioligand. RESULTS: There was selective radioligand binding in the kidneys, mainly located in the cortex. Within the time interval between 95 and 115 min postinjection, the radioactivity retained in the kidneys was 109 +/- 27 and 42 +/- 4 nCi/ml/mCi of the injected dose for the control and inhibition studies, respectively. The influx rate constant of the radioligand decreased from a baseline of 0.0298 +/- 0.0156 to a post-MK-996 value of 0.0098 +/- 0.0052. CONCLUSION: These results demonstrate distinct binding of 11C-L-159,884 in the renal cortex with a specific binding component suitable for quantitative PET imaging of angiotensin II/AT1 receptors.

Positron emission tomography of 5-HT reuptake sites in the human brain with C-11 McN5652 extraction of characteristic images by artificial neural network analysis.

The goal of this work was to identify the distribution of serotonin transporters in the human brain with [11C](+)McN5652/PET. Four healthy volunteers were studied. To determine non-specific binding, a PET study was also performed with the inactive enantiomer [11C](-)McN5652 as well as with [11C](+)McN5652 after pretreatment with fluoxetine. For pattern extraction the PET data sets were analyzed by a back-propagation neural network. Two pharmacokinetic patterns and two characteristic images were separated; one representing specific binding, the other representing non-specific binding. The specific binding image showed characteristic distribution of serotonin transporters with [11C](+)McN5652. The pattern images demonstrated an improvement in image quality compared to the original PET images (reduced variance, higher region-to-cerebellum ratio, good correlation with known density of serotonin transporters). The non-specific binding images extracted from [11C](-)McN5652/PET were similar to those of [11C](-)McN5652 and [11C](-)McN5652 with fluoxetine. Thus, PET studies obtained with [11C](+)McN5652 largely represent the regional distribution of the serotonin transporters and the inactive enantiomer [11C](-)McN5652 shows the distribution of its nonspecific binding.

A simple probe measures the pharmacokinetics of[125I]RTI-55 in mouse brain in vivo.

A simple external radiation detector system was used to assess brain dopamine and serotonin transporters in mice in vivo using [125I]RTI-55. The results were compared to ex vivo dissection data. Methyl 3beta-(4-iodophenyl) tropane-2beta-carboxic acid methyl ester (RTI-55 or beta-CIT), a high-affinity cocaine antagonist, binds to presynaptic dopamine and serotonin transporters in the brain. Radiotracer accumulation increased for the first 150 min after intravenous injection and then remained constant. When unlabeled RTI-55 was injected, either before or 60 min after radiotracer administration, a significant decrease in tracer accumulation was observed. [125I]RTI-55 binding was also displaced by blocking doses of 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenylpropyl]piperazine dihydrochloride (GBR 12909) and paroxetine. The results were similar to the ex vivo dissection data. The results demonstrate the feasibility of using the probe detector system to study the presynaptic transporter system in vivo in the mouse brain. The technique is applicable to other cerebral neurotransmitter systems.

[11C]-methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl)-methyl]-1- piperazinecarboxylate ([11C]GR89696): synthesis and in vivo binding to kappa opiate receptors.

GR89696, racemic methyl 4-[(3,4-dichlorophenyl)acetyl]-3-[(1-pyrrolidinyl) methyl]-1-piperazinecarboxylate, a kappa opioid receptor ligand, was labeled with [11C]methyl chloroformate. The radiochemical yield was 20% with an observed specific radioactivity of 75.5 GBq/micromol at end of synthesis (2,040 mCi/micromol). Five minutes after intravenous administration, 5.4% of the injected dose accumulated in mouse whole brain. Brain region to cerebellar ratios increased over time with ratios at 90 min of 7.8, 5.6, and 4.5 for the hypothalamus, olfactory tubercle, and striatum, respectively. The uptake of [11C]GR89696 correlated with known kappa opioid receptor densities and was inhibited by kappa opioid selective drugs.

Selective alkylation of pyrimidyl dianions III: no-carrier-added synthesis of [11C-methyl]-thymidine.

A no-carrier-added, high specific activity synthesis of [11C-methyl]-thymidine is reported. Reaction of 3'. 5'-O-bis-(tetrahydropyramyl)-5-bromo-2'-deoxyuridine with n-butylithium produced a diamion which was alkylated with [11C]-methyl iodide, and on subsequent hydrolysis, yielded [IIC-methyl]-thymidine. The labeled compound was isolated from the by-product 2'-deoxymidine by HPLC on a reverse phase C18 semipreparative column with mean radiochemical yield of 18.8% (decay corrected) in 30-35 min and radiochemical purity >99%. This no-carrier-added synthesis can be used to produce [11C-methyl]-thymidine with mean specific activity over 1000 mCi/mumol for positron emission tomography (PET) studies.

An improved method for the synthesis of radiolabeled McN5652 via thioester precursors.

An improved procedure that facilitates routine production and increases the radiochemical yield of [11C]McN5652 (trans-1,2,3,5,6,10b-hexahydro-6-[4-([11C]methylthio)-phenyl]pyrrolo- [2,1-alpha]-isoquinoline) is presented. Specifically, thiol acetate, butyrate, and benzoate derivatives of McN5652 were prepared as the precursors for the [11C]McN5652 synthesis. These thioesters offer greater stability than the previously used thiol precursor (desmethyl McN5652) and enable a single batch of material to be used for multiple radiolabelings. Hydrolysis of the thioester functionality (tetrabutylammonium hydroxide, 10 min) unmasked the free thiol which, without purification, was reacted with [11C]iodomethane in DMF at 40-45 degrees C for 1 min. The average decay-corrected radiochemical yield for [11C]McN5652 was 26% with an average specific activity of 2290 mCi/mumol (end of synthesis). This facile radiolabeling method, utilizing the butyrate thioester of McN5652, was also employed in the preparation of [3H](+)- and (-)McN5652 [trans-1,2,3,5,6S (or 6R),10bR, (or 10bS)-hexahydro-6-[4-([3H]methylthio)phenyl]pyrrolo-[2,1,alpha]- isoquinoline] from [3H]iodomethane.

[11C]A-69024: a potent and selective non-benzazepine radiotracer for in vivo studies of dopamine D1 receptors.

[11C]A-69024, (+/-)-1-(2-bromo-4,5-dimethoxybenzyl)-7-hydroxy-6-methoxy-2-[11C]methyl- 1,2,3,4-tetrahydroisoquinoline, is a specific and selective dopamine D1 radiotracer. The in vivo biodistribution of this novel radioligand in mice showed a high uptake in the striatum (6.7% ID/g) at 5 min, followed by clearance with a half-life of 16.1 min. As a measure of specificity, the striatal/cerebellar ratio reached a maximum of 7.4 at 30 min post-injection. Radioactivity in the striatum was reduced to the level of the cerebellum by pre-administration of the D1 antagonist SCH 23390 (1 mg/kg). Pretreatment of mice with spiperone (D2), 7-hydroxydipropylaminotetralin (7-OH-DPAT) (D3), clozapine (D4), ketanserin (5-HT2/5-HT2C), mazindol (monoamine reuptake), prazosin (alpha 1), and haloperidol (D2/sigma) had no inhibitory effect on [11C]A-69024 uptake in the striatum. The dextrorotatory enantiomer of the dopamine antagonist butaclamol inhibited striatal uptake, while the less active isomer (-)-butaclamol did not. [11C]A-69024 binding was inhibited by unlabeled A-69024 in a dose dependent manner (ED50 = 0.3 mg/kg) in the striatum while no change occurred in the cerebellum. [11C]A-69024 warrants further investigation as a PET ligand for examination of central dopamine D1 receptors in humans.

Synthesis of an I-123 analog of A-85380 and preliminary SPECT imaging of nicotinic receptors in baboon.

A radiosynthetic method to prepare the nicotinic acetylcholine receptor radioligand (S)-5-[123I]iodo-3-(2-azetidinylmethoxy)pyridine, 5-IA, has been developed. The two-step sequence produced [123I]-5-IA in high radiochemical yield (52%), high radiochemical purity (98%), and high specific radioactivities (> 8,500 mCi/mumol). Preliminary single photon emission computed tomography studies with [123I]-5-IA in baboon demonstrated the appropriate regional localization for a high-affinity nicotinic radioprobe (thalamus > frontal cortex > cerebellum). Pretreatment with cytisine blocked [123I]-5-IA uptake in all brain regions (78-59% reduction), demonstrating the specificity of the radiotracer.

[125/123I] 5-Iodo-3-pyridyl ethers. syntheses and binding to neuronal nicotinic acetylcholine receptors.

Three 3-pyridyl ether nicotinic ligands-(S)-5-Iodo-3-[(2-pyrrolidinyl)-methoxy]pyridine (5-iodo-A-85865), (S)-5-Iodo-3-[1-(methyl)-2-pyrrolidinyl-methoxy]pyridine (5-Iodo-A-84543), and (S)-5-iodo-3-[1-methyl-(2-azetidinyl)-methoxy]pyridine (5-iodo-N-Me-A-85380) were labeled with I-125/I-123, and their ability to label high-affinity brain nicotinic acetylcholine receptors (nAChRs) was evaluated. The most promising ligand, [123/125I] 5-iodo-A-85865, showed approximately 65% inhibition of radioactivity uptake in thalamus in mice pretreated with cytisine. Preliminary SPECT imaging studies with [123I] 5-iodo-A-85865 revealed a distribution profile consistent with nAChRs (thalamus > frontal cortex > cerebellum) and a more rapid pharmacokinetic profile relative to azetidinyl 3-pyridyl ether based ligands.

In vivo imaging of nicotinic receptor upregulation following chronic (-)-nicotine treatment in baboon using SPECT.

To quantify changes in neuronal nAChR binding in vivo, quantitative dynamic SPECT studies were performed with 5-[(123)I]-iodo-A-85380 in baboons pre and post chronic treatment with (-)-nicotine or saline control. Infusion of (-)-nicotine at a dose of 2.0 mg/kg/24h for 14 days resulted in plasma (-)-nicotine levels of 27.3 ng/mL. This is equivalent to that found in an average human smoker (20 cigarettes a day). In the baboon brain the regional distribution of 5-[(123)I]-iodo-A-85380 was consistent with the known densities of nAChRs (thalamus > frontal cortex > cerebellum). Changes in nAChR binding were estimated from the volume of distribution (V(d) ) and binding potential (BP) derived from 3-compartment model fits. In the (-)-nicotine treated animal V(d) was significantly increased in the thalamus (52%) and cerebellum (50%) seven days post cessation of (-)-nicotine treatment, suggesting upregulation of nAChRs. The observed 33% increase in the frontal cortex failed to reach significance. A significant increase in BP was seen in the thalamus. In the saline control animal no changes were observed in V(d) or BP under any experimental conditions. In this preliminary study, we have demonstrated for the first time in vivo upregulation of neuronal nAChR binding following chronic (-)-nicotine treatment.

2-[18F]Fluoro-A-85380, an in vivo tracer for the nicotinic acetylcholine receptors.

The in vivo brain regional distribution of 2-[18F]fluoro-A-85380, a novel tracer for positron emission tomographic (PET) studies, followed the regional densities of brain nAChRs reported in the literature. Evidence of binding to nAChRs and high specificity of the binding in vivo was demonstrated by inhibition with nAChR selective ligands as well as with unlabeled 2-fluoro-A-85380. A preliminary toxicology study of the 2-fluoro-A-85380 showed a relatively low biological effect. 2-[18F]Fluoro-A-85380 holds promise as a useful radiotracer for imaging of nAChRs with PET.

(+)-[C-11]-cis-N-benzyl-normetazocine: a selective ligand for sigma receptors in vivo.

The in vivo biodistribution profile of the novel sigma (sigma) receptor ligand (+)-[C-11]-cis-N-benzyl-normetazocine ([C-11]-(+)-NBnNM) in mouse brain was examined. This radioligand displayed high brain uptake and a distribution consistent with the density of sigma receptors. Brain radioactivity levels peaked at 15 min postinjection and were largely maintained (ca. 80% of maximal values) up to 90 min postinjection. Pretreatment with several different sigma ligands (haloperidol, (+)-pentazocine, DuP 734, ifenprodil) effectively inhibited [C-11]-(+)-NBnNM binding in a dose-dependent manner in all brain regions. [C-11]-(+)-NBnNM binding sites were shown to be saturable with unlabeled (+)-NBnNM (ED50 = 0.02 mg/kg) and enantioselectively inhibited by the optical isomers of pentazocine. A blocking dose of the dopamine D2 antagonist spiperone (1 mg/kg) did not significantly inhibit [C-11]-(+)-NBnNM binding. Pretreatment with the phencyclidine (PCP) blocker 1-[1-(2-thienyl)cyclohexyl] piperidine (TCP) did not significantly alter total brain tissue radioactivity. Thus, [C-11]-(+)-NBnNM binds with high specificity and selectivity to sigma receptors in vivo and offers excellent potential to study sigma receptors in living human brain via positron emission tomography.

Radioiodinated D-(+)-N1-ethyl-2-iodolysergic acid diethylamide: a ligand for in vitro and in vivo studies of serotonin receptors.

Radioiodinated D-(+)-N1-ethyl-2-iodolysergic acid diethylamide ([125I]-EIL) has been evaluated as a ligand for in vitro and in vivo studies of cerebral serotonin 5-HT2 receptors. [125I]-EIL exhibited high affinity (KD = 209 pM) for 5-HT2 receptors with a high degree of specific binding (80-95%) in membranes from rat prefrontal cortex. The regional distribution of [125I]-EIL binding in vivo to seven areas of mouse brain correlated significantly (Rs = 0.93) with known densities of 5-HT2 receptors. In vivo specificity, defined by tissue to cerebellum radioactivity ratios, reached a maximum for frontal cortex at 6 hr (21.2) and persisted through 16 hr (8.8). Ketanserin, a 5-HT2 receptor antagonist, fully inhibited binding in a dose dependent fashion in all brain regions except cerebellum. By contrast, blockers for dopamine D2, alpha- or beta-adrenergic receptors did not significantly inhibit radioligand binding in any region. [125I]-EIL selectively labels 5-HT2 receptors in vivo with the highest specificity of any serotonergic ligand reported to date, indicating that [123I]-EIL should prove applicable to single photon emission computed tomography studies in living brain.

In vitro and in vivo binding of (E)- and (Z)-N-(iodoallyl)spiperone to dopamine D2 and serotonin 5-HT2 neuroreceptors.

Apparent affinities (Ki) of (E)- and (Z)-N-(iodoallyl)spiperone [E)- and (Z)-NIASP) for dopamine D2 and serotonin 5-HT2 receptors were determined in competition binding assays. (Z)-NIASP (Ki 0.35 nM, D2; Ki 1.75 nM, 5-HT2) proved slightly more potent and selective for D2 sites in vitro than (E)-NIASP (Ki 0.72 nM, D2; Ki 1.14 nM, 5-HT2). In vivo, radioiodinated (E)- and (Z)-[125I]-NIASP showed regional distributions in mouse brain which are consonant with prolonged binding to dopamine D2 receptors accompanied by a minor serotonergic component of shorter duration. Stereoselective, dose-dependent blockade of (E)-[125I]-NIASP uptake was found for drugs binding to dopamine D2 sites, while drugs selective for serotonin 5-HT2, alpha 1-adrenergic and dopamine D1 receptors did not inhibit radioligand binding 2 hr postinjection. Specific binding in striatal tissue was essentially irreversible over the time course of the study, and (E)-[125I]-NIASP gave a striatal to cerebellar tissue radioactivity concentration of 16.9 to 1 at 6 hr postinjection. Thus, (E)-[125I]-NIASP binds with high selectivity and specificity to dopamine D2 sites in vivo.

[3H]A-69024: a non-benzazepine ligand for in vitro and in vivo studies of dopamine D1 receptors.

[3H]A-69024 has been prepared as a radioligand for studying the dopamine D1 receptor. [3H]A-69024 binds to rat striatal membranes with a KD = 14.3 +/- 3.2 nM (mean +/- SEM; n = 3) and Bmax = 63.5 +/- 12.8 fmol/mg wet tissue (1.8 +/- 0.3 pmol/mg protein). This ligand binds to only one site with a Hill coefficient close to unity. The in vivo biodistribution of [3H]A-69024 showed a high uptake in the striatum (5.9% ID/g) at 5 min followed by clearance. As a measure of specificity, the striatum/cerebellar ratio reached a maximum of 6.7 at 30 min post-injection. Pre-treatment with the D1 antagonist R(+)SCH 23390 (1 mg/kg) reduced this ratio to unity. The dopamine antagonist (+)butaclamol and unlabeled A-69024 inhibited striatal uptake by 70 and 51%, respectively. Spiperone (D2/5-HT2A) and ketanserin (5-HT2A/5-HT2C) at doses of 1 mg/kg had no inhibitory effect on [3H]A-69024 uptake in the striatum; however, increased uptake of [3H]A-69024 by > 30% in the whole brain was observed. The selectivity and affinity of [3H]A-69024 suggests that this non-benzazepine radioligand may be useful for in vitro and in vivo studies of the dopamine D1 receptor.

5-[I-125/123]lodo-3(2(S)-azetidinylmethoxy)pyridine, a radioiodinated analog of A-85380 for in vivo studies of central nicotinic acetylcholine receptors.

The in vivo biodistribution profile of the novel nicotinic acetylcholine receptor (nAChR) radioligand 5-[I-125/123]Iodo-3(2(S)-azetidinylmethoxy)pyridine, [I-125/123]-5-IA, in mouse brain was examined. This radiotracer displayed good brain penetration (3.1% of the injected dose (ID) in whole brain at 15 min post-radioligand injection). Radioligand distribution was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus (14.9 %ID/g at 60 min), moderate uptake in cortex (8.5 %ID/g at 60 min), and lowest uptake in the cerebellum (2.4 %ID/g at 60 min). Pretreatment with several different nAChR agonists (A-85380, (-)-nicotine, cytisine) significantly inhibited [I-125]-5-IA binding in all brain regions studied (P < 0.01) demonstrating the high specificity of the radioligand for nAChRs. Blocking doses of the muscarinic antagonist scopolamine and the non-competitive nAChR channel blocker mecamylamine had no significant effect on radioactive uptake supporting the in vitro selectivity of [I-125]-5-IA for the nAChR component of the cholinergic system. [I-125]-5-IA binding sites were shown to be saturable with unlabeled 5-IA. With a relatively low acute toxicity (LD50 > 3 mg/kg via intravenous injection in mice) and high in vivo specificity and selectivity, 5-IA labeled with the imaging radionuclide I-123 may prove useful for single photon emission computed tomography (SPECT) studies of nAChRs in human subjects.

Pharmacological evaluation of [11C]A-84543: an enantioselective ligand for in vivo studies of neuronal nicotinic acetylcholine receptors.

[11C]A-84543, 3-[(1-[11C]methyl-2(S)-pyrrolidinyl)methoxy]pyridine, is a specific and enantioselective neuronal nicotinic acetylcholine receptor (nAChR) radiotracer. The in vivo biodistribution of this radiotracer in mice showed high brain uptake and a distribution consistent with the density of nAChRs. Highest uptake was observed in the thalamus (9.6 %ID/g), cortex (9.9 %ID/g), superior colliculus (7.6 %ID/g) and hippocampus (7.6 %ID/g) at 5 min followed by clearance. As a measure of specificity, the thalamus/cerebellar ratio reached a maximum of 2.3 at 30 min post-injection. Radioactivity in the thalamus and superior colliculus was reduced by 33% by pre-administration of unlabeled A-84543. The nAChR agonists (-)nicotine, cytisine, and (+) epibatidine reduced the radioactivity due to [11C]A-84543 in the superior colliculus by 41%, 38%, and 27%, respectively, while lobeline, which also interacts with central nAChRs, produced a 24% inhibition. The noncompetitive nAChR ligand, mecamylamine displayed no inhibitory effect on [11C]A-84543 accumulation in any brain region. Ketanserin (5-HT2/5-HT2C), scopolamine (mAChR antagonist), (+)butaclamol (DA receptor antagonist), and haloperidol (D2/sigma) also displayed no inhibitory effect in any brain region studied. With the pharmacologically less active enantiomer, 3-[(1-[11C]methyl-2(R)-pyrrolidinyl)methoxy] pyridine, high brain uptake was also observed, but with a low thalamus/cerebellar ratio of 1.4 at 30 min post-injection. [11C]A-84543 displays enantioselectivity for nAChRs and may deserve further investigation as a possible PET radiotracer.

3 Beta-(p-trimethylsilylphenyl)tropane-2 beta-carboxylic acid methyl ester: a new precursor for the preparation of [123I]RTI-55.

Here we report the synthesis of 3 beta-(p-trimethylsilylphenyl)tropane-2 beta-carboxylic acid methyl ester, a new, easily prepared precursor for the synthesis of radioiodinated versions of RTI-55. In contrast to the 3 beta-(p-trimethylstannylated) precursor for radioiodination, the aryl silane is prepared directly by the 1,4 addition of p-trimethylsilylphenylmagnesium iodide to anhydroecgonine methyl ester in 49% yield (Scheme 1). Radioiododesilylation proceeded smoothly (approx. 85% radioincorporation after 15 min at room temperature) using 1 microgram of the novel precursor in trifluoroacetic acid and chloramine-T as oxidant. Isolated radiochemical yields for 125I and 123I radiolabelings were 72 and 48%, respectively, with a calculated specific activity for [123I]RTI-55 of > 2000 mCi/mumol.